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NASA said the ice deposits were patchily distributed and “could be ancient”. (File)

On Tuesday, US space agency NASA said a team of scientists, while studying data generated by an instrument that had travelled on India’s Chandrayaan-1 spacecraft in 2008, had directly observed “definitive evidence” of water ice on the Moon’s surface. NASA said the ice deposits were patchily distributed and “could be ancient”.

“M3 (NASA’s Moon Mineralogy Mapper) aboard the Chandrayaan-1 spacecraft… was uniquely equipped to confirm the presence of solid ice on the Moon. It collected data that not only picked up the reflective properties we’d expect from ice, but was able to directly measure the distinctive way its molecules absorb infrared light, so it can differentiate between liquid water or vapour and solid ice,” NASA said in a statement.

While NASA’s announcement is based on the results of a new study, published in the Proceedings of the National Academy of Sciences, this is not the first time that evidence of water has been found on the Moon. Nor is it the first time that the contribution of Chandrayaan-1 is being acknowledged in establishing such evidence.

First confirmation

In September 2009, data from the same M3 instrument was used to announce “unambiguous evidence” of presence of water across the lunar surface. That announcement was made after the data of M3 had been corroborated by observations of NASA’s EPOXI spacecraft as it passed the Moon on its way to comet Hartley 2, and a reassessment of the data produced by a spectrometer aboard Cassini spacecraft in 1999. The data from ISRO’s hyperspectral imager, an instrument used for mapping minerals, also aboard Chandrayaan-1, supplemented the evidence.

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Distribution of surface ice (blue) at Moon’s south (left) and north poles, detected by NASA’s M3 instrument on board Chandrayaan-1. (Source: NASA)

That announcement was seen as the final confirmation of water on Moon, something that had been hypothesised since the first lunar missions in the 1960s and 1970s. Water molecules were found mostly in the polar regions of the Moon.

Days after that announcement, ISRO said another of its instruments on Chandrayaan-1, the Moon Impact Probe or MIP, had produced compelling evidence of water on the Moon. ISRO said this evidence was available at least a few months before NASA’s announcement. MIP, a 35-kg cube-shaped instrument with the Tricolour on all sides, is the first Indian object to land on the Moon.

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G Madhavan Nair, ISRO chairman at the time of Chandrayaan-1 launch, explained why the agency could not publish the findings of the MIP: “There was an instrument inside MIP that was supposed to assess the moisture content in Moon’s surface. It gave us distinct signature (of the presence of water). But unfortunately, there were some calibration anomalies in the data because of which we could not have quantified the amount of water content. As a result, we were not in a position to publish the results.” Nair said data of the hyperspectral imager, however, was very useful in establishing the presence of water.

Reconfirmation

After 2009, several studies have pointed to the presence of water, in different forms. Most of these have used the same data sets as used for the 2009 announcement.

In August 2013, for example, a team of US scientists looked at the same M3 data and detected magmatic water, “or water that originates within the Moon’s interior”, on the lunar surface. A NASA statement said it represented the “first detection of this form of water from lunar orbit”.

Last year, another team used M3 data and produced the first map of water distribution on the lunar surface, showing it was spread across the Moon and not just in polar regions.

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In February this year, NASA reported a new analysis of data from two lunar missions that presented fresh evidence of water being “widely distributed” across the surface. It said the water appeared to be “present day and night, though it is not necessarily easily accessible”.

The latest finding, that of solid ice, takes these discoveries forward. “Most of the newfound water ice lies in the shadows of craters near the poles, where the warmest temperatures never reach above -250°F (-150°C). Because of the very small tilt of the Moon’s rotation axis, sunlight never reaches these regions,” the NASA statement said.